Re: [Boost-bugs] [Boost C++ Libraries] #10639: lexical_cast<double>(string) wrong in C++11

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #10639: lexical_cast<double>(string) wrong in C++11
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-10-10 17:38:59

#10639: lexical_cast<double>(string) wrong in C++11
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  Reporter:  richard.kreckel@…  |      Owner:  apolukhin
      Type:  Bugs               |     Status:  new
 Milestone:  To Be Determined   |  Component:  lexical_cast
   Version:  Boost 1.56.0       |   Severity:  Problem
Resolution:                     |   Keywords:
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Changes (by johnmaddock):

 * cc: pbristow@… (added)

Comment:

 I had a quick look at this, and can confirm the issue - I'm really pretty
 surprised to see that the code is parsing the number itself rather than
 relying on std::num_get or whatever.  Here's the thing: you absolutely
 cannot do this correctly (base 10 to 2 conversion) without arbitrary
 precision arithmetic - indeed in the worst case there is basically no
 limit to how many digits you may need in order to carry out the conversion
 correctly (though such cases are extremely rare, as in they will never
 ever occur in practice!).  In fact getting this right is really bloody
 hard - Boost.Multiprecision has an algorithm under
 boost/multiprecision/cpp_bin_float/io.hpp based on MPFR's largely brute
 force approach, but honestly I wouldn't use that either for convert-to-
 built-in-type.  There's more information at
 http://www.exploringbinary.com/real-c-rounding-is-perfect-gcc-now-

 converts-correctly/ which shows that even many respected std lib's often
 don't get this right in certain cases.  I'm adding Paul Bristow to the CC
 list, as I know he has an interest in this.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/10639#comment:1>
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